The present invention relates to a hydraulic fluid pressure control system for a transmission, and more particularly relates to a hydraulic fluid pressure control system for a transmission which is an overdrive or a subtransmission coupled to a main transmission, and which has two speed stages which are selected by selective supply of a hydraulic fluid pressure.
In a copending U.S. patent application Ser. No. 193,752 filed Oct. 3, 1980, which has been filed by the same applicants, and assigned to the same assignee, as the present application, there is described a transmission system which comprises a main transmission device which is a gear selection type transmission device which may be manually shifted between a plurality, usually three or more, of transmission speed stages, and which is coupled to a subtransmission or overdrive transmission device, which is a planetary gear type transmission device of separate construction from and attached to the main transmission device, and which is selectable, independently of the main transmission device, between a high speed stage and a low speed stage, which in most cases are an increased speed stage and a direct connection speed stage.
Further, in the abovementioned copending patent application, there is proposed such a transmission in which the selection thereof between its two speed stages is performed by selective supply of hydraulic fluid pressure to a hydraulic fluid actuator comprised therein. This selective supply of hydraulic fluid pressure, according to the operating conditions of the vehicle incorporating the transmission, and of the engine driving it, may be performed automatically, in order to reduce the difficulty of operation of the automobile by the driver thereof.
A problem in prior art transmission systems has been that a line hydraulic fluid pressure used for various functions within the transmission control system has been maintained at a substantially constant level, whatever use is being made of this line hydraulic fluid pressure. Thus, a high line pressure was maintained, even when no substantial use was being made of this line pressure for operating fluid actuators for friction engaging elements of the transmission. This has caused undue strain upon various operating components of the transmission such as the hydraulic fluid pressure pump thereof, and also has resulted in undue use of energy for pressurizing hydraulic fluid up to this line pressure, when supply of such pressurized hydraulic fluid is not required.
Further, in the case of a subtransmission which is electrically controlled, and in which switching of hydraulic fluid pressure is performed by a switching valve which is controlled by a solenoid, because typically a shifting member of the switching valve has been directly shifted by the solenoid, such a solenoid has had to be relatively bulky, and has caused a substantial power drain upon the electrical system of the automobile. Further, heavy duty relays are required for the control of such a solenoid, if the solenoid is controlled by a computer such as a microcomputer, as is nowadays frequently the case.